Rinnai RX160 vs RX180 vs RX199: Sizing a Tankless by Bathrooms

Short answer: As a rough starting point for homes across the GTA and Southern Ontario, the Rinnai RX160 (9.0 GPM max) suits 1 to 2 bathrooms, the RX180 (10.0 GPM) suits 2 to 3, and the RX199 (11.1 GPM) suits 3 or more or homes with a large soaker tub. Cold Ontario winter water reduces real-world flow, so size for winter, not summer.

The three models at a glance

All three are condensing tankless heaters from Rinnai's RX series. They share the same basic design and features, and differ mainly in how much gas they can burn and how much hot water they can deliver at once.

Specs are from Rinnai's product pages for the natural gas models (RX160iN, RX180iN, RX199iN). Each has a minimum input of 15,000 BTU/h, so they can turn down for a single small tap. The "rough fit" column is our general guidance, not a Rinnai rating. Your household's habits matter more than the bathroom count alone.

Why the max GPM isn't what you'll get in January

The maximum flow rating is reached only when the water coming in needs very little heating. A tankless heater's real limit is how much it can raise the water temperature at a given flow.

Across the GTA and Southern Ontario, water coming into your home is much colder in winter than in summer. Colder incoming water means the heater has to do more work on every litre, so it can deliver fewer litres per minute at shower temperature. A unit that easily runs two showers in August may struggle to run two showers plus the dishwasher in February.

That's why we size for the coldest month and the busiest few minutes of your day, not for the headline number.

How to size for your home

Think about the most hot water your household uses at the same moment. For most families that's the morning rush.

1. Count what runs at once. Two showers and a bathroom sink? One shower and the washing machine?
2. Check your fixtures. Most showerheads have their flow rating in GPM or L/min stamped on them. Rain heads, body sprays and large tub fillers can use a lot more than a standard head.
3. Add it up. Total the flow of everything that runs at once, then remember winter will reduce what the heater can deliver.
4. Leave some headroom. If you're between two sizes, the larger one gives you margin for guests, a future bathroom, or a bigger tub.

When to choose each model

Rinnai RX160

• Condos, townhomes and smaller detached homes
• One or two people showering at a time
• The lowest installed price in the range, at $5,495 before HST

Rinnai RX180

• Typical family homes with two to three bathrooms
• Two showers running while the kitchen or laundry draws hot water
• A middle ground when the RX160 feels tight and the RX199 feels like more than you need

Rinnai RX199

• Larger homes, three or more bathrooms, or a big soaker tub
• Households where several people shower at the same time
• The top of our tankless range at $6,045 installed, before HST

Gas line and venting matter as much as size

A bigger heater needs more gas when it fires. Before we install, we check that your gas meter and piping can supply the model you choose. If they can't, we quote the gas line work before we start. We also plan the vent route and a drain for the condensate these units produce.

These checks are why a tankless quote sometimes includes extras that a tank swap doesn't. You'll know about them up front.

Frequently asked questions

Which Rinnai tankless do I need for 2 bathrooms?

Many two-bathroom homes do well with the RX160 or RX180. If two showers often run at the same time as a dishwasher or laundry, the RX180 gives more margin in winter.

What's the difference between the RX160, RX180 and RX199?

Mainly capacity. Rinnai rates them at 160,000, 180,000 and 199,000 BTU/h maximum input and 9.0, 10.0 and 11.1 GPM maximum flow. The RX160 is rated 0.97 UEF; the RX180 and RX199 are rated 0.98.

Can I run two showers at once on a tankless?

Yes, if the unit is sized for it. The key is winter performance, when colder incoming water reduces how much hot water any tankless heater can deliver.

Is tankless right for my home at all?

If you're still deciding, read our tank vs tankless guide. A tank can be the better value if upfront cost is your main concern.

Shop Rinnai tankless water heaters · Financing · Ask us to size it or call 905-418-2629

Specs from Rinnai's product pages for the RX160iN, RX180iN and RX199iN, checked September 2026. Prices are installed prices for standard installation across the GTA and Southern Ontario, before HST.